1. Field of the Invention
This invention relates to apparatus for water and wastewater treatment. More specifically, the invention relates to filter underdrain blocks used to provide even distribution of liquid and gas during filter backwashing operations and also the uniform collection of filtered water during the normal operation of filtration units.
2. Description of the Related Art
It is desirable to provide water and wastewater filtration units with a means for supporting the filter media such that the filter media is spaced apart from the bottom of the filter. This provides a space for the pipes which comprise that portion of the backwash air distribution system which is situated below the filter media. It also physically separates the filter media from the air distribution pipes. This helps prevent clogging of the orifices in the pipes by the finer particles of the filter media as well as removing the weight load of the overlying filter media from the pipes. Physical abrasion of the pipes is also avoided in this type of filter apparatus inasmuch as the gravel of the filter media is not in contact with the walls of the pipes. During backwashing operations the particles comprising the filter media tend to move about to some extent in response to the force of backwash air and water flows. If the filter media is in direct physical contact with the backwash air distribution pipes the pipes will be subjected to the abrasive action of the moving filter media (typically gravel and sand). Such abrasion shortens the useful life of the air distribution system.
Prior to the present invention, one of the most effective means for supporting filter media in the manner described above was to equip the filter with precast concrete underdrain blocks. These are specially designed blocks cast using high-strength concrete which are placed side-by-side and end-to-end in the bottom of a filter so as to form a "false bottom" which provides a support for the filter media. The blocks have legs such that when the blocks are placed on the bottom surface of the filter, passages are provided for the air distribution pipes between the legs of the blocks.
In the past, such filter underdrain blocks had to be cast in metal molds which must be partially disassembled to remove the block after the concrete was sufficiently cured and then reassembled to cast the next block. The molds themselves were expensive and the production of such filter underdrain blocks in commercial quantities required the use of multiple molds inasmuch as one had to wait for the concrete to set on one block before that mold became available to cast another block. A typical cycle time for such a mold is 24 hours. Moreover, once the blocks were cast at the casting facility they had to be shipped to the filter construction site for use. This was expensive since the blocks are heavy and it subjects the blocks to potential damage during shipment and associated handling.